How do you read a GD&T feature control frame?

What does each compartment of a feature control frame mean?

The first compartment holds the geometric characteristic symbol, the second holds the tolerance (zone shape, value and modifiers), and any further compartments name the datum features in order of precedence. A frame with no datum compartments is a form control, or a control that does not need a datum reference frame.

Take this frame, written as a row of compartments:

[ ⌖ | Ø0.25 Ⓜ | A | B | C ]

Read aloud, it says: the position of this feature (or each feature in this pattern) must lie within a cylindrical zone of diameter 0.25, which applies when the feature is at its maximum material condition and grows by the bonus as the feature departs from it, and the zone is located by basic dimensions from datum reference frame A, B, C, with A primary, B secondary and C tertiary. The units are those of the drawing, so on a metric drawing per ASME Y14.5-2018 that zone is a quarter of a millimeter across at MMC.

Every part of that sentence came from a specific compartment. Change any one of them, such as dropping the diameter symbol, swapping B and C, or removing the circled M, and the requirement on the part changes, often by enough to move a good part to the reject bin or the other way round.

What do the modifiers in a feature control frame change?

Modifiers change when the stated tolerance applies or where the zone lives. Without any modifier, ASME Y14.5-2018 applies the tolerance regardless of feature size and references datum features regardless of material boundary.

• Circled M (maximum material condition). The stated value applies at MMC, and the zone grows by the difference between the feature's actual mating size and MMC. It is the modifier that makes functional gauging possible. The arithmetic is worked through in the MMC bonus tolerance guide.
• Circled L (least material condition). The mirror image: the value applies at LMC and grows toward MMC. It protects minimum wall thickness, for example around a bore near a cast edge.
• Circled P (projected tolerance zone). The zone is projected above the part, by the height stated after the symbol or in the drawing, so it controls the axis where a mating stud or press-fit pin will actually sit. It belongs on threaded holes and press-fit holes whose axis tilt would otherwise cause interference in the mating part, as covered in the threaded-hole position guide.
• Circled F (free state). The tolerance applies with the part unrestrained, used on nonrigid parts that a separate note says are inspected in a restrained condition.
• Circled T (tangent plane). Only the plane contacting the high points of the surface must lie in the zone, not every point of the surface.
• Material boundary modifiers on datums. A circled M or circled L after a datum letter means maximum or least material boundary for that datum feature of size. It lets the part shift on the datum simulator, which is datum feature shift, not bonus tolerance.

Two consequences follow for reading the frame. First, a circled M after the tolerance does nothing to the datums; each datum letter carries its own modifier, or none. Second, with no circled M or circled L after a datum feature of size, the inspector must center the part on that feature with an adjustable simulator, such as an expanding mandrel or a fixture that closes on it, because the datum is referenced at regardless of material boundary.

Which feature control frames must, may, or must not reference datums?

Form controls never reference datums, runout always does, and orientation and location controls need datums to mean anything. Profile may be used with or without datums, and the difference is large.

Flatness, straightness, circularity and cylindricity describe a feature's shape against itself, so a frame for any of them has two compartments only. If you see a datum letter in a flatness frame, the drawing has an error; the designer probably meant parallelism. Circular and total runout always reference a datum axis, because they are measured by rotating the part about it; the distinction between the two is in the runout guide.

Perpendicularity, parallelism and angularity need at least one datum, since orientation is relative to something. Position needs datums except in a few special cases defined by the standard. Profile of a surface without datums controls the shape of the surface but does not orient or locate it to the rest of the part; with datums it also controls orientation and location, which is why a single profile frame on a casting can replace many dimensions, as explained in the profile tolerance guide.

Concentricity and symmetry no longer exist in ASME Y14.5-2018. If an older drawing carries them and is revised to the 2018 edition, the frames are usually replaced with position, runout or profile, and the concentricity, runout and position guide compares the options.

What should a drawing and RFQ say so a supplier reads every feature control frame the same way?

Name the standard and its edition in the title block, attach datum feature symbols to real features, give basic dimensions for every toleranced location, and flag the frames that matter most. Many disputes over a feature control frame come from a drawing that left one of those open.

• Edition. Write "Dimensioning and tolerancing per ASME Y14.5-2018" or the ISO equivalent. Defaults differ between editions and between ASME and ISO, as the ASME vs ISO guide explains.
• Datum features. Each letter in a frame must be identified on the drawing by a datum feature symbol or by datum targets. On a casting, datum targets on the raw surfaces are a common choice for the first setup.
• Basic dimensions. Position and profile zones are located by boxed basic dimensions from the datums. A location given as a plus-minus dimension instead is a conflict to resolve before quoting.
• Critical frames. Mark the frames tied to fit or function, so inspection effort goes there.
• Inspection method. Say whether a functional gauge, a coordinate measurement or a fixture is expected for frames with material modifiers, since the gauge design follows from the frame (gauge principles are in ASME Y14.43).

When Saola receives a drawing, the quality page describes what happens next: every tolerance, callout and note is read and returned as a marked-up copy of your own drawing, and the first-article report measures every toleranced dimension on the first piece from new tooling, with a dimensional inspection record for each production lot. Dimensions you mark as process-capability control points are checked more often through the run. On machined castings, the CNC milling page lists the bore and GD&T tolerances held on machined castings and states that every GD&T callout is measured and logged; simultaneous five-axis machining is not offered. Turned features run on CNC turning. ISO 9001 is in progress, not yet held. Send the drawing with its edition note through the quote form, and see the RFQ guide for the rest of the package.